1. Field of the invention
This invention pertains generally to pressurized compartments and is more particularly concerned with cabin pressure control systems for pressurized passenger aircraft cabins.
2. Description of the prior art
Over the years there has been a steady improvement in performance of cabin pressure control systems for the larger commercial aircraft. With design perfection many problems have been eliminated and other problems have been reduced significantly. Accordingly it was inevitable, in considering the overall advantages of aircraft flight at greater altitudes, that the manufacturers of the smaller private and so-called executive class of aircraft would establish a trend to the pressurization of their aircraft cabins.
Known cabin pressure control systems have not proven to be satisfactory since they are too elaborate and expensive for such reduced requirements. Also, mere scaling down in size has been found to be an unsatisfactory procedure, probably because it is not possible to scale down the important element of time as well as that of dimensions. For example, the time lag between the instant of sensing a pressure change and a later time when the system correction has been made may be a very critical factor in aircraft passenger comfort. In the larger aircraft where large space volumes are to be contended with, the time lag can be longer than in the case of a smaller space volume. That is, where the space volume is large, transient pressure excursions due to inflow transients or changes in atmosphere pressure tend to smooth out, and passenger discomfort is at a minimum.
However, in the smaller aircraft it has been found that the mere act of preparing to take off, for example, together with the takeoff run and the subsequent operation of landing gear retraction after the wheels leave the ground, can subject the passengers to the severe discomfort of an annoying cabin "bump." For example, in a laboratory test to simulate the takeoff of a Cessna 500 aircraft from a simulated field altitude 2,250 ft., with the aircraft having a current cabin pressure control system, it was demonstrated that the cabin received a positive pressure "bump" equivalent to 310 ft. in altitude within about 10 seconds after start of takeoff, and then receded in pressure altitude to a "negative" bump of about 332 ft. from that artificial altitude in the next seven seconds. It will be noted that the negative bump was much more severe in rate of change of altitude than was the positive bump.
In order to overcome to some extent the effects of such system operation it has been necessary for the crew of the aircraft to carefully ascertain and select the field altitude on the cabin pressure controller prior to take-off, and then to reselect the cruise cabin altitude following lift-off. This, of course, contributes to crew work load at a critical time without assurance that a random pressure transient won't negate the initial setting. The requirements for reselection of cabin cruise altitude means further that another cabin pressure transient is experienced at that time.
The cabin pressure regulating system currently in use, as noted above, does not include a pneumatic relay in its operation, although other systems do employ such a component to provide amplification and stabilization of outflow valve drive. Consequently, in attacking the problems as aforesaid it was considered that a starting point for their solution might be to add such a relay. In the course of such routine engineering study of the design problem, the inspiration was had to change the control pressure signal for the relay from the usual location in communication with the referenced chamber pressure of the controller to the memory or rate chamber pressure. The system gain provided by the relay permits relocation of control to memory. This was done and the discovery was made that unusual improvements resulted as follows:
a. Cabin transients as a result of the reselection of cabin altitudes in flight are eliminated.
b. The system error as a function of aircraft pressure differential and cabin inflow level is reduced by approximately 50 percent. This improves the selectability of the system.
c. The system dynamic response to cabin inflow variations has also been improved by approximately 50 percent. This reduces cabin pressure excursions due to cabin inflow or outflow variations caused by other equipment.